Mesophyll and bundle sheath chloroplasts have been isolated and purified from corn leaves which have higher photochemical activity when DPIP, ferricyanide, of NADP+ is used as electron acceptor. Furthermore, BS chloroplasts are active in NADP+ photoreduction without addition of exogenous cofactor. Rate of Hill reaction with either ferricyanide of DPIP as electron acceptor is 2 to 3 times higher in the presence of methylamine in spinach and corn mesophyll chloroplasts. However, methylamine stimulates ferricyanide reduction to a lesser extent in BS chloroplasts and practically has no effect on DPIP reduction. These results suggest that PS I and Ps II are active and linked in both mesophyll and BS chloroplasts. Mesophyll cells have been isolated from two C3 plants, spinach and Ipomoea aquatica. 14CO2 fixation of the isolated cells is assayed. The effect of 2,4,5-T, CPIC, simazine, and diuron on photosynthetic 14CO2 fixation by Ipomoea aquatica cells is examined and compared to their effect of Hill reaction of isolated chloroplasts. Simazine and diuron give similar inhibitory effect of 14CO2 fixation by plant cells and Hill reaction by chloroplast. In contrast, 2,4,5-T causes a stimulation on 14CO2 fixation by cells and an inhibition on Hill reaction by isolated chloroplasts.